Detailed knowledge of the groundwater flow regime is a pre-condition for understanding the geochemical[unreadable] processes in the subsurface. Identification of recharge and discharge areas and mechanisms, as well as flow[unreadable] lines, are essential for delineating trends in geochemical evolution?including changes in As concentrations,[unreadable] and to quantify reaction rates. The Hydrogeology Support Core will provide the tools and expertise for[unreadable] collection and analysis of a broad range of hydrogeological data for Projects 5, 6 and 7, and the Research[unreadable] Translation Core. The following types of data will be obtained and interpreted: 1) longitude, latitude, and[unreadable] elevation; 2) stratigraphy of the aquifer from core samples and drill cuttings, hydraulic head, conductivity,[unreadable] storativity, and porosity data; 3) geophysical data, primarily frequency EM conductivity, and down-hole logs[unreadable] of resistivity, gamma, flow velocity, and televiewer images; and 4) tracer data (SF6, CH4, Br, 3H/3He, 13C, 14C,[unreadable] 13C, 18O, 2H). The equipment to be provided and operated by the Core include: 1) Multilevel wells, 2) a[unreadable] Geoprobe and other push coring and sampling devices, 3) in situ groundwater monitoring devices, 4) a[unreadable] frequency EM conductivity system, 5) various down-hole logging tools, 6) sample preparation systems for[unreadable] isotopic analyses, 7) gas chromatographs, and 8) noble gas mass spectrometers. Carbon (13C, 14C) and[unreadable] water isotope (18O,2H) analyses will be performed by commercial laboratories. The Core will provide support[unreadable] for the various push/pull and forced gradient in situ experiments, with a focus on characterizing the zone of[unreadable] injections and tracking injected fluids with tracers. Tracer data will also be used to derive groundwater[unreadable] residence times for a range of time scales (from months to 10,000s of years), to identify recharge and[unreadable] discharge areas and mechanisms and to track groundwater mixing in the aquifers. The research support[unreadable] core will provide three-dimensional modeling capabilities for groundwater flow and reactive transport[unreadable] simulations. We will use hydraulic and tracer data as calibration targets for the model groundwater flow and[unreadable] transport model, which will then provide the baseline for reactive transport modeling aided with[unreadable] biogeochemical data obtained by the Biogeochemistry Core.